Classical hemophilia A is the result of a chromosome X-linked deficiency of blood plasma coagulation factor VIII and affects almost exclusively males with a frequency of about 1 case per 10,000. The X-chromosome defect is transmitted by female carriers who do not themselves have the disease. Factor VIII is also known as antihemophilic factor (AHF), hemophilic factor A, platelet cofactor, thromboplastinogen, thrombocytolysin, and antihemophilic globulin (AHG). The designation "factor VIII:C" is used to indicate that it is the compound that affects clotting. Factor VIII is a high molecular weight protein of 280 kDa and is composed of two polypeptide chains of 200 kDa and 80 kDa, respectively. Andersson et al., Proc. Natl. Acad. Sci. U.S.A., 83:2979-2973 (1986). These chains are held together by a metal ion bridge.
The principal symptom of hemophilia A is bleeding without clotting or coagulation. Prior to the discovery that administration of factor VIII concentrates could ease the symptoms of an individual diagnosed with the disease, the average life expectancy of a sufferer was about 20 years.
Until recent years, the major source of factor VIII for therapeutic purposes was normal blood plasma; however factor VIII isolated by this method, while of some use, has several important disadvantages. For instance, factor VIII isolated from blood plasma is fairly impure, typically having a specific activity of less than 2 units factor VIII/mg protein and an overall factor VIII content of less than 1%. Additionally, the purification process is expensive because the starting material, i.e., human plasma, is expensive. Many precautions must also be taken to decrease the risk of transmitting infectious agents to the patient. For example, human immunodeficiency virus (HIV), Hepatitis B virus, Hepatitis C virus and other disease-causing agents are commonly detected in donated blood. Another disadvantage of using factor VIII obtained by this method is that approximately one-tenth of the patients with severe hemophilia A develop antibodies against factor VIII, making the disease difficult to treat.
Research efforts have focused on the development of methods for creating and isolating highly purified, biologically active factor VIII in full-length and derivative forms. Advantages of a highly purified protein include reduced levels of extraneous proteins in the therapeutic mix as well as a decreased likelihood of the presence of infectious agents. A more purified form of factor VIII may also be administered in smaller doses, possibly reducing the risk of developing anti-factor VIII antibodies, as lower doses would be less challenging to the immune system.
Significant steps have been taken toward the recombinant production of factor VIII beginning with the isolation of biologically active factor VIII fragments. See, Andersson et al., U.S. Pat. No. 4,749,780; Andersson et al., U.S. Pat. No. 4,877,614. The gene encoding the full-length human factor VIII protein was isolated by taking advantage of its sequence homology with porcine factor VIII. See, Toole et al., U.S. Pat. No. 4,757,006. Toole et al. also report the expression of human and porcine protein having factor VIII:C procoagulant activity.
However, severe side effects involving the production of anti-factor VIII antibodies still exist with the administration of the protein isolated from both human and non-human sources. Antibodies that react with human factor VIII:C are also known to react, to a certain extent, with factor VIII:C from other species, and porcine factor VIII itself is antigenic in humans. Also, non-hemophiliacs can develop or acquire the disease when their immune systems become sensitized to factor VIII:C.
As a possible solution to this problem, a truncated, lower molecular weight protein exhibiting procoagulant activity has been designed. See, Toole, U.S. Pat. No. 4,868,112. Toole reported an alternative method of treatment with lower molecular weight porcine factor VIII of approximately 2000 amino acids exhibiting similar procoagulant activity as full-length factor VIII. Evidently, the removal of certain amino acids and up to 19 of the 25 possible glycosylation sites, reduced the antigenicity of the protein and thereby the likelihood of developing anti-factor VIII antibodies. However, one difficulty with the development of recombinant factor VIII is achieving production levels in sufficiently high yields.
Recently, deleted factor VIII cDNA molecules coding for recombinant factor VIII derivatives, which were likely to give sufficiently high yields of a biologically active recombinant factor VIII protein for use in an industrial process for a pharmaceutical preparation have been developed. See, Almstedt et al., U.S. Pat. No. 5,661,008. Almstedt et al. designed a modified factor VIII derived from a full-length factor VIII cDNA, that, when expressed in animal cells, produced high levels of a factor VIII-like protein with factor VIII activity. The protein consisted essentially of two polypeptide chains derived from human factor VIII, the chains having molecular weights of 90 kDa and 80 kDa, respectively.
According to the Almstedt et al. process, the factor VIII cDNAs are assembled into transcription units and introduced into a suitable host system for expression. The cell lines can be grown on a large scale in suspension culture or on solid support. The protein produced in the culture medium is then concentrated and purified. The final active protein is made up of amino acids 1 to743 and 1638 through 2332 of human factor VIII This polypeptide sequence is commercially known as rFVIII-SQ or REFACTO.RTM.. See also, Lind et al., Euro. J. Biochem., 232:19-27 (1995). Other forms of truncated FVIII can also be constructed in which the B-domain is generally deleted. In such embodiments, the amino acids of the heavy chain, consisting essentially of amino acids 1 through 740 of human Factor VIII and having a molecular weight of approximately 90 kD are connected to the amino acids of the light chain, consisting essentially of amino acids 1649 to 2332 of human Factor VIII and having a molecular weight of approximately 80 kD. The heavy and light chains may be connected by a linker peptide of from 2 to 15 amino acids, for example a linker comprising lysine or arginine residues, or alternatively, linked by a metal ion bond.
Currently, there is a need in the field for efficient and cost-effective methods for obtaining purified, active factor VIII directly from various solutions such as blood or cell culture supernatants.
The present invention provides new materials and methods for identifying, isolating, and purifying factor VIII and factor VIII-like proteins, including REFACTO.RTM., from a solution that contains such proteins, in an active form. The factor VIII binding molecules of the present invention exhibit high affinity for factor VIII and factor VIII-like peptides. The current invention thus provides a cost-effective means for rapid purification of commercial quantities of proteins useful in the treatment of hemophilia A.